1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and more particularly to the field of marine seismic surveys. Specifically, the invention is an apparatus for attenuating noise in marine seismic streamers.
2. Description of the Related Art
In the field of geophysical prospecting, knowledge of the subsurface structure of the earth is useful for finding and extracting valuable mineral resources, such as oil and natural gas. A well-known tool of geophysical prospecting is a seismic survey. In a seismic survey acoustic waves are transmitted from appropriate energy sources into the earth and the reflected signals are collected using an array of sensors. Seismic data processing techniques are then applied to the collected data to estimate the subsurface structure.
In a seismic survey, the seismic signal is generated by injecting an acoustic signal from on or near the earth's surface, which then travels downwardly into the subsurface of the earth. Appropriate energy sources may include explosives or vibrators on land and air guns or marine vibrators in water. When the acoustic signal encounters a seismic reflector, an interface between two subsurface strata having different acoustic impedances, a portion of the acoustic signal is reflected back to the surface, where the reflected energy is detected by a sensor.
Appropriate types of seismic sensors include particle velocity sensors in land surveys and water pressure sensors in marine surveys. However, particle acceleration sensors may be used instead of particle velocity sensors. Particle velocity sensors are commonly known in the art as geophones and water pressure sensors are commonly know in the art as hydrophones. Both seismic sources and seismic sensors may be deployed by themselves or, more commonly, in arrays.
In a typical marine seismic survey, a seismic vessel travels on the water surface, typically at about 5 knots, and contains seismic acquisition equipment, such as navigation control, seismic source control, seismic sensor control, and recording equipment. The seismic source control equipment causes a seismic source towed in the body of water by the seismic vessel to actuate at selected times. The seismic source may be of any type well known in the art of seismic acquisition, including airguns or water guns, or most commonly, arrays of airguns. Seismic streamers, also called seismic cables, are elongate cable-like structures towed in the body of water by the seismic survey vessel that tows the seismic source or by another seismic survey ship. Typically, a plurality of seismic streamers are towed behind a seismic vessel. The seismic streamers contain sensors to detect the reflected wavefields initiated by the seismic source and reflected from reflecting interfaces. Conventionally, the seismic streamers contain pressure sensors such as hydrophones, but seismic streamers may also contain water particle motion sensors such as geophones. The sensors are typically located at regular intervals along the seismic streamers.
Seismic streamers are typically divided into sections approximately 100 meters in length, and can extend to a length of thousands of meters. Position control devices such as depth controllers, paravanes, and tail buoys are used to regulate and monitor the movement of the seismic streamers. Seismic data gathering operations are becoming progressively more complex, as more sources and streamers are being employed. A common feature of these source and streamer systems is that they can be positioned astern of and to the side of the line of travel of the seismic vessel. The sources and streamers are submerged in the water, with the seismic sources typically at a depth of 5-15 meters below the water surface and the seismic streamers typically at a depth of 5-40 meters.
FIG. 1 is a side sectional view of a portion of a marine seismic streamer section. A typical streamer section includes an outer skin 11, strength members 12, spacers 13, and an electrical wire bundle 14. The outer skin 11 protects the interior of the streamer section from water ingress. Connectors (not shown) at the ends of each streamer section link the section mechanically, electrically and/or optically to adjacent sections and, hence, ultimately to the seismic towing vessel. The strength members 12, usually two or more, run down the length of each streamer section from end connector to connector, providing axial mechanical strength. Strength members 12 are typically made of fiber rope, such as Vectran fiber, which is a registered trademark of Hoechst Chemical Corp., New York. The wire bundle 14 also runs down the length of each streamer section, and includes electrical power conductors and data communication wires. In some instances, fiber optics for data communication are included in the wire bundle 14. Sensors 15, typically hydrophones or groups of hydrophones, are located within the streamer. The hydrophones 15 have sometimes been located within the spacers 13 for protection. The distance between spacers 13 is normally about 0.7 meters. A group of hydrophones 15, typically comprising 8 or 16 hydrophones 15, normally extends for a length of about 12.5 meters.
FIG. 1 is a schematic representation of a portion of the streamer section. In this representation, the outer skin 11 and strength members 12 are illustrated as straight and uniformly thick throughout the length of the streamer section. The outer skin 11 and strength members 12 are designed to be straight and uniform while they are not subject to external stress. The electrical wire bundle 14, on the other hand, is designed to be curved and nonuniform, so that it can deform readily as the streamer cable bends while turning in the water, or is being deployed from or retrieved onto the streamer winch for storage on the survey vessel.
The interior of the seismic streamers is filled with a core material 16 to provide buoyancy and desirable acoustic properties. For many years, most seismic streamers have been filled with a fluid core material 16. However, there are two main drawbacks with this type of design. The first drawback is leakage of the fluid into the surrounding water when a streamer section is damaged and cut. Since the fluids in the streamers are typically hydrocarbons, such as kerosene or light oil, this leakage may be an environmental problem. This damage can occur while the streamer is being towed through the water or it can occur while the streamer is being deployed from or retrieved onto the streamer winch on which streamers are typically stored on the seismic tow vessel.
The second drawback to using fluid-filled streamer sections is the noise generated by vibrations as the streamer is towed through the water. These vibrations develop internal pressure waves traveling through the fluid in the streamer sections, which are often referred to as “bulge waves” or “breathing waves”. This noise is described, for example, in the paper S. P. Beerens et al., “Flow Noise Analysis of Towed Sonar Arrays”, UDT 99—Conference Proceedings Undersea Defense Technology, Jun. 29-Jul. 1, 1999, Nice, France, Nexus Media Limited, Swanley, Kent.
FIG. 2 is a side sectional view of a portion of a streamer section experiencing bulge wave noise. In the ideal situation of a streamer moving at constant speed, the components of the streamer—the outer skin 11, strength members 12, spacers 13, wire bundle 14, connectors, and fluid core material 16—all move in unison and do not move relative to each other. In realistic conditions, however, vibrations of the seismic streamer leading to transient motion of the strength members 12 are caused by such events as pitching and heaving of the seismic vessel, paravanes, and tail buoys attached to the streamers; strumming of the towing cables attached to the streamers caused by vortex shedding on the cables, or operation of depth-control devices located on the streamers. The arrow 17 designates the direction of tow in FIG. 2, which is also the direction in which the strength members 12 are being irregularly pulled. The transient motion of the strength members displaces the spacers 13 or connectors, causing pressure fluctuations in the fluid core material 16 that are detected by the hydrophones 15. The pressure fluctuations radiating away from the spacers 13 or connectors also cause the flexible outer skin 11 to neck in, as depicted at 21, and bulge out, as depicted at 22, as a traveling wave. The outer skin 11 is no longer straight as in the ideal situation depicted in FIG. 1, above. The bulging 22 of the outer skin 11 in this traveling wave gives this phenomenon one of its names—bulge waves.
In addition, there are other types of noise, often called flow noise, which can affect the hydrophone signal. For example, vibrations of the seismic streamer can cause extensional waves in the outer skin 11 and resonance transients traveling down the strength members 12. A turbulent boundary layer created around the outer skin 11 of the streamer by the act of towing the streamer can also cause pressure fluctuations in the fluid core material 16. The extensional waves, resonance transients, and turbulence-induced noise are typically much smaller in amplitude than the bulge waves. Bulge waves are usually the largest source of vibration noise because these waves travel in the fluid core material 16 filling the streamer sections and thus act directly on the hydrophones 15.
There are several ways to reduce the noise problem in fluid filled steamer sections. For example, a first approach is to introduce compartment blocks in the sections to impede the vibration-caused bulge waves from traveling continuously along the streamer. The noise can be isolated and thus attenuated. A second approach is to introduce open cell foam into the interior cavity of the streamer section. The open cell foam restricts the flow of the fluid fill material 16 in response to the transient pressure change and causes the energy to be dissipated into the outer skin 11 and the foam over a shorter distance. A third approach to address the noise problem is to combine several hydrophones 15 into a group to attenuate a slow moving wave. An equal number of hydrophones 15 are positioned between or on both sides of the spacers 13 so that pairs of hydrophones 15 sense equal and opposite pressure changes. Summing the hydrophone signals from a group can then cancel out some of the noise.
Another approach to eliminating the bulge waves in the fluid core material 16 is to eliminate the fluid from the streamer sections, so that no medium exists in which bulge waves can develop. This approach is exemplified by the use of so-called solid streamers, using streamer sections filled with a solid core material 16 instead of a fluid. However, in any solid type of material, some shear waves will develop, which can increase the noise detected by the hydrophones 15. (Note that shear waves cannot develop in a fluid fill material 16 since fluids have no shear modulus.) Additionally, many conventional solid core materials 16 are not acoustically transparent to the desired pressure waves.
A further approach to solving the noise problem is to replace the fluid core material 16 in a streamer section with a softer solid core material 16. The introduction of a softer solid material 16 may block the development of bulge waves, as compared to a fluid core material 16. A softer solid material 16 may also attenuate the transmission of shear waves, as compared to a harder solid core material 16. However, there can still be a substantial transmission of shear waves through the softer solid material 16 to the hydrophones 15. One approach to this problem of shear waves is described in co-pending U.S. patent application Ser. No. 11/059,497, “Apparatus for Attenuating Noise in Marine Seismic Streamers”, filed on Feb. 16, 2005, assigned to the assignee of the present application and having as a co-inventor the inventor of the present application. This application describes a marine seismic streamer, a hydrophone housing positioned in the marine seismic streamer, the hydrophone housing having ends and substantially rigid side walls, a hydrophone positioned in the hydrophone housing, a soft compliant solid material filling the housing and the marine seismic streamer, and openings in the hydrophone housing adapted to substantially permit passage of pressure waves and to substantially attenuate passage of shear waves.
Using a soft compliant material 16 may eliminate most of the problem with bulge waves, but noise from longitudinal transient waves traveling down the strength members 12, caused by the so-called Poisson effect, will still be present. Thus, a need exists to further improve the attenuation of noise caused by longitudinal waves transmitted through the strength members 12 in seismic streamers.